Radio frequency attenuator



Jan. 15, 1963 J. F. HENl-:Y 3,073,990

RADIO FREQUENCY ATTENUATOR Filed June 25, 1958 @gula/IJ? Agen United States Patent O 3,073,990 RADIO FREQUENCY ATTENUATGR John F. Heney, Bloomeld, NJ., assigner to International Telephone and Telegraph Corporation, Nutley, NJ., a corporation of Maryland Filed June 23, 1958, Ser. No. 743,676 4 Claims. (Qi. 315-39) This invention relates to radio frequency transmission line attenuators and more particularly to electron discharge attenuators for employment in radio frequency transmission lines. a With the advent of long range, high speed aircraft, it 1s necessary to increase the range of obstacle detection systems to enable identification and maximum opportunity for interception. The range of obstacle detection systems may be increased by the use of much higher power and lower frequency radiated energy than has heretofore been required. This approach has brought about changes in the components required for these systems and has brought to light the fact that components which were considered conventional in the lower power, high frequency systems are not suited for high power operation or are not operable at the lower frequencies used in these relatively low frequency systems. This critique is particularly applicable to attenuating or switching devices such as gas tube TR and ferrite switches.

Gas tube TR devices are limited in their use as far as high power operation is concerned in that there is a limit to the amount of radio frequency power which can be employed. If this radio frequency power limit is exceeded, losses in the switch can destroy the tube or seriously shorten tube life. Ferrite type switches are limited in their utility in that they are known to have a low frequency limit at which their non-reciprocal characteristics disappear and they are limited in their power handling capacity.

The structures described and claimed herein are referred to as attenuators, either fixed or variable, to better define the scope of this invention. A switch, such as a conventional TR device, is considered to be a fixed attenuator having substantially infinite reactive attenuation when in a conducting or operative condition.

It is therefore, an object of this invention to provide a radio frequency transmission line attenuator which overcomes the limitations of prior art devices.

Another object of this invention is to provide a radio frequency transmission line attenuator which is operable under high power, low frequency conditions.

A further object of this invention is to provide a novel 'coaxial line radio frequency attenuator.

Still a further object of this invention is to provide a novel coaxial line radio frequency attenuator which is leither a switch or variable attenuator.

A feature of this invention is the provision of a radio frequency attenuator including a transmission line conductor having two ends defining a gap therebetween, an electrode disposed in juxtaposition to each of said ends land means to selectively energize said electrodes to proto diameter of the center conductor of a coaxial line to provide a conductive path between the ends of a gap lin the center conductor.

Still another feature of this invention is the provision of a source of variable power which, when applied to the "ice cathode, acts to vary the plasma frequency of an electron discharge across a gap disposed in the center conductor of a coaxial line in a manner to vary the attenuation of the radio frequency energy being conducted across the conductor gap by said electron discharge.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a longitudinal cross sectional view of a coaxial line attenuator in accordance with the principles of this invention.

FIG. 2 is a graphical representation of attenuation vs. plasma frequency illustrating the relationship between the attenuation of a radio frequency signal in decibels (db) and the plasmo frequency of an electron beam.

Referring to FIG 1, there is shown therein a section of coaxial line 1 having an outer conductor 2 and a hollow inner conductor 3. Inner conductor 3 has a gap 4 disposed therein which is defined by the ends 5, 6 of inner conductor 3. Two electrodes 7, 8 are shown in juxtaposition to ends 5, 6 of center conductor 3 and means are shown which permit the selective energization of electrodes 7, S to provide an electron discharge between these electrodes toV provide a conductive path for radio frequency energy from source 9 to load 9 across the gap 4. The electrodes 7, 8 and the gap 4.are shown to be hermetically sealed by dielectric envelope 10 to enable the establishment of a relatively high vacuum electron discharge across gap 4.

In FIG. l, electrodes 7, 8 which are an electron emissive electrode or cathode and a collector, respectively, are disposed in gap 4 of inner conductor 3 in juxtaposition to the ends 5, 6 of inner conductor 3. The cathode 7 is shown as the type which emits a hollow annular beam of electrons 11. It should be noted, at this point, that while the use of a hollow beam is preferable, one need not be limited by this fact. A solid cylindrical beam may be used, but the diameter of the beam, whether it is hollow or solid, should have a diameter which substantially approaches the diameter of inner conductor 3. By this means, large discontinuities between the ends 5, 6 of inner conductor 3 and the beam 11 are avoided and lower insertion loss can be expected. The hollow beam, however, has been found to be preferable not only because the electrons on the outer surface of the beam are the only ones which contribute to the conduction of radio frequency energy across the gap but also by reason of lower heater requirements and better collector cooling obtained.

The cathode 7 and collector element 8 are shown in FIG. 1 disposed perpendicular to the longitudinal axis of coaxial transmission line 1. Cathode 7, which is heated by filament 12, is further shown displaced from gap end 5 by a small distance d to insure that hollow annular electron beam 11 provides a conductive path across gap 4. Cathode 7 is shown in FIG. 1 as an indirectly heated cathode but it may be directly heated without affecting the operation of this device.

In FIG. l, a third electrode, focusing and accelerating anode 13 is shown disposed in the path of electron beam 11 and contains an annular cutout portion 14 to permit passage of hollow annular beam 11 therethrough. Support wires 1S which may be tungsten or some other metal which can withstand high temperatures are connected to portion 16 of electrode 13 to support the central portion 13a of electrode 13 in its desired coaxial relationship. Portion 16 of electrode 13 extends beyond the dielectric envelope 10 which acts to maintain a vacuum around electrodes 7, 8, 13 and filament 12. The extended portion 16 of electrode 13 acts as a flange against which center conductorend 5 is firmly butted thereby maintaining electrode 13 at the same potential as that of center conductor 3. Collector electrode 8 likewise contains a flanged portion 16a which extends beyond dielectric envelope 10. This flanged portion 16a is butted firmly ,against center conductor end 6 and this flange in cooperation with the ange on end S of center conductor 3 holds the electrode assembly in fixed relation to center conductor 3. Collector electrode 8, in order to dissipate heat rapidly, may be cooled by water or some other well known coolant and in FIG. 1, ports for the ingress and egress of coolant are designated 17, 1S, respectively. 'The coolant means may be introduced into center conductor 3 by the well known means of a matching stub which is not shown in the drawing.

Cathode 7 is heated by filament 12, the power to filament 12 being supplied by source 19. Cathode 7 is selectively energized for electron emission by means of pulser which .can be adjusted to provide pulses of any amplitude and duration. Pulser 20, therefore, can supply pulses in the range of microsecond duration up to and including continuous wave operation. VTo provide a pulsed electron beam, a negative potential of given amplitude and duration from pulser 2t? is applied to cathode 7. Beam focusing means 21, which may be a solenoid for applying a magnetic field parallel to the longitudinal axis of the tube, maintains electron beam 11 in a given path as it traverses gap 4. Beam focusing means 21 may also be electrostatic in nature and would use a number of electrodes with proper applied voltages to maintain electron beam 11 in a given path.

In operation, as a substantially infinite attenuator or switch, pulser Ztl applies a negative pulse of given amplitude, for a given duration, to cathode 7 which cause the `|cathode to emit an electron beam 11 which is accelerated across the gap d and caused to drift at a constant velocity 'across the distance l to impinge on collector electrode "8. In order to insure proper operation of this device when the electron beam is acting as a conductor for radio 'frequency across the gap, the plasma frequency of the electron beam must be substantially hgher than the fre- 'quen'ry of the radio energy vbeing transmitted across the gap and the electrons must drift across the distance l at a constant velocity. The plasma frequency of an electron Vbeam amy be defined as the frequency of radiation due to the motion of electrons oscillating about their point of -least energy. A more complete exposition on this subject may be found in the hook Gaseous Conductors, by I. D. Cobne, p. 131, first edition, McGraw-Hill Book Co. If these criteria are not adhered to, the radio fre- 'quency energy attempting to pass the gap by means of the electron beam 11 will be attenuated. The plasma `frequency is directly proportional to the power applied -by variable power pulser 20 and whenthe device is acting as a switch, a constant amount of power, sufiicient to cause the plasma frequency to be substantially higher than the radio frequency, is applied. When the puiser 20 removes the negative potential from cathode 7, electron emission Vstops and the conductive path across gap 4 is removed. Radio frequency energy being transmitted along coaxial line 1 from source 9 upon reaching gap 4 is reactively attenuated, i.e reflected, and this attenuation increases exponentially as a function of the gap length designated l in FIG. 1. From this, it may be seen that the gap length l may be set to a predetermined point and a fixed amount of attenuation introduced other than the substantially infinite attenuation which is introduced when the attenuator is being used as a switch.

When the device'of FIG. 1, as just described, is being used as a switch or as a fixed attenuator, the plasma frequency of the electron beam 11 must be substantially higher than the frequency of the energy being carried by coaxial line 1. For switching and introducing a fixed amount of attenuation, the radio frequency energy must encounter a large discontinuity which is provided when the gap 4 iS dVQid 0f the conductive electron beam 11. This means that radio frequency energy will be transmitted without attenuation to load 9 when puiser 20 energizes cathode 7. Thus, during the pulser of= time, attenuation, either substantially infinite or of fixed value dependent on gap length l, is introduced and, as in prior art devices such as gas discharge coaxial switches, no means for changing the attenuation during the conduction cycle has been provided. However, it has been found, using the device of PIG. 1, that variable attenuation can be obtained during the conduction cycle by varying the power of variable pulser 20 to vary the plasma frequency of electron beam 11.

FIG. 2 is a graphical representation of attenuation vs. plasma frequency illustrating the relationship between the attenuation of radio frequency signal in db as the plasma frequency of the electron beamis increased. Referring to FG. 2, it may be seen that as the plasma frequency increases the attenuation decreases from point A to practically 0 db at point B. Investigation has shown that if the gap length l is adjusted such that 40 db reactive attenuation, for instance, is present when there is no conductive electron beam present, it is possible by applying increasing power to pulser 20 to gradually decrease the attenuation of an RE. signal in accordance with the curve of FIG. 2. With the filament power constant to make electrons available for instantaneous operation, the power to pulser 20 is increased from zero, electrons are emitted from cathode 7 and are accelerated across gap d at a relatively low value of plasma frequency. At point A, where the plasma frequency is in the lorder of one-tenth the frequency of the radio frequency energy, the attenuation starts to decrease. As the plasma frequency in increased by increasing the power to cathode 7, the attenuation continues to decrease until poin B is reached where a further increase in plasma frequency provides no further decrease in attenuation. At point B the plasma frequency is greater than ten times the frequency of the radio energy being transmitted. Thus, by varying the amplitude of the pulses, for instance,

from pulser 20, it is possible to obtain variable attenuation of radio frequency energy during the conducting cycle. This type of attenuation is particularly useful at frequences of the order of 20 megacycles and is only limited in utility when it is not practical to obtain the required electron densities necessary to keep the plasma frequency high with respect to the signal frequency being used.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. A radio frequency variable attenuator comprising a coaxial line having an inner and outer conductor for transmission of radio frequency energy therealong, said inner conductor having two ends defining a fixed gap therebetween, a cathode disposed at one end of said gap, a collector disposed at the other of said ends, heater means causing said cathode to emit electrons, beam forming means to provide a longitudinal hollow beam of electrons between said cathode and said collector, a dielectric vacuum envelope to maintain a vacuum between said cathode and said collector and adjustable pulse means coupled to said cathode to variably control the power and plasma frequency of said electron beam and the attenuation of said gap to said radio frequency energy.

2. A radio frequency attenuator comprising a coaxial line having an inner and outer conductor for transmission of radio frequency energy therealong, said inner conductor having two ends defining a predetermined gap therebetween, a first electron emission electrode disposed at one of said ends, a second collector electrode forming the other of said ends, means heating said first electrode to emit electrons, beam forming means to provide a longitudinal focused electron beam between said first and second electrodes, a third electrode disposed in the plane of and adjacent said rst end to provide said beam with a constant diameter and to accelerate said electron beam to a given velocity, means coupled to said first electrode to provide a source of adjustable pulsed power to Vary the plasma frequency and attenuation of said electron beam and a vacuum envelope to maintain a vacuum between said first and second electrodes.

3. A radio frequency attenuator comprising a coaxial line having an inner and outer conductor for transmission of radio frequency energy therealong said inner conductor having two ends defining a fixed gap therebetween, a first electrode disposed at one of said ends, a second electrode disposed in the plane of the other of said ends, means to provide an electron discharge in the form of a focused, cylindrical electron beam between said first and second electrodes along a given path collinear with and having substantially the same diameter as said inner conductor, a third electrode disposed in the plane of said one of said ends between said first and second electrodes to provide said beam with a constant diameter and to accelerate said electron beam to a given velocity, a vacuum envelope to maintain a vacuum between said rst and second electrodes and variable pulsed power means coupled to said first electrode to adjustably control the plasma frequency of said electron discharge to a higher frequency than said radio frequency and to vary the value of attenuation of said radio frequency energy.

4. A radio frequency variable attenuator comprising a coaxial line having an inner and outer conductor for transmission of radio frequency energy therealong, said inner conductor having two ends defining a fixed gap therebetween, a cathode disposed at one of said ends, a collector disposed in the plane of the other of said ends,

means to provide a focused cylindrical, electron beam between said cathode and said collector as a collinear eX- tension of said inner conductor, a beam forming anode disposed in the plane of said one of said ends between said cathode and said collector to provide said beam with a constant diameter and to accelerate said electron beam to a given velocity, a dielectric vacuum envelope to maintain a vacuum between said cathode and said collector and means applied to said cathode to provide a variable amplitude and duration pulse to adjustably control the plas-ma frequency of said electron beam with respect to the radio frequency and thereby, to Vary the value of attenuation of said radio frequency energy.

References Cited in the file of this patent UNITED STATES PATENTS 2,153,728 Southworth Apr. 1l, 1939 2,416,168 Fiske Feb. 18, 1947 2,557,961 Goldstein et al. June 26, 1951 2,643,297 Goldstein et al. June 23, 1953 2,703,882 Wilkes Mar. 8, 1955 2,765,421 Robertson et al. Oct. 2, 1956 2,791,711 Harris May 7, 1957 2,795,760 Dench June 11, 1957 2,820,172 Field Jan. 14, 1958 2,826,718 Larson et al Mar. 11, 1958 2,870,374 Papp Jan. 20, 1959 2,953,713 Geiger Sept. 20, 1960 

1. A RADIO FREQUENCY VARIABLE ATTENUATOR COMPRISING A COAXIAL LINE HAVING AN INNER AND OUTER CONDUCTOR FOR TRANSMISSION OF RADIO FREQUENCY ENERGY THEREALONG, SAID INNER CONDUCTOR HAVING TWO ENDS DEFINING A FIXED GAP THEREBETWEEN, A CATHODE DISPOSED AT ONE END OF SAID GAP, A COLLECTOR DISPOSED AT THE OTHER OF SAID ENDS, HEATER MEANS CAUSING SAID CATHODE TO EMIT ELECTRONS, BEAM FORMING MEANS TO PROVIDE A LONGITUDINAL HOLLOW BEAM OF ELECTRONS BETWEEN SAID CATHODE AND SAID COLLECTOR, A DIELECTRIC VACUUM ENVELOPE TO MAINTAIN A VACUUM BETWEEN SAID CATHODE AND SAID COLLECTOR AND ADJUSTABLE PULSE MEANS COUPLED TO SAID CATHODE TO VARIABLY CONTROL THE POWER AND PLASMA FREQUENCY OF SAID ELECTRON BEAM AND THE ATTENUATION OF SAID GAP TO SAID RADIO FREQUENCY ENERGY. 